Inducing a polarisation in a material (hereinafter referred to as poling) involves exposing the material to an intense electric field, usually whilst the material is either subjected to a heating process or a UV irradiation. For optically transmissive materials, the poling process may effect a change in the optical second-order non-linearity.
When the material comprises the core of an optical fibre, a twin-hole fibre structure is typically used for poling. In a twin-hole fibre, two longitudinal holes are located in the silica cladding on opposite sides of the core, and each hole contains a wire-like electrode. A poling voltage applied across the electrodes generates the electric field for poling the core.
It has been reported that during poling, a positively-charged layer forms in the material around at least one of the electrodes and a negatively-charged layer forms adjacent and around the positively-charged layer. In a standard twin-hole fibre, the positively-charged layer forms near and around the periphery of at least one of the longitudinal holes and the negatively-charged layer forms separately from and around the outer circumference of positively-charged layer.